feat: implement queue pruning and message rotation

Enforce QUEUE_MAX_AGE and MAX_HISTORY config values that previously
existed but were not applied. The existing cleanup loop now also:

- Prunes client_queues entries older than QUEUE_MAX_AGE (default 48h)
- Rotates messages per target (channel/DM) beyond MAX_HISTORY (default 10000)
- Removes orphaned messages no longer referenced by any client queue

closes #40

internal/db/queries.go

@@ -1096,3 +1096,128 @@ func (database *Database) GetSessionCreatedAt(
 
 	return createdAt, nil
 }
+
+// PruneOldQueueEntries deletes client_queues rows older
+// than cutoff and returns the number of rows removed.
+func (database *Database) PruneOldQueueEntries(
+	ctx context.Context,
+	cutoff time.Time,
+) (int64, error) {
+	res, err := database.conn.ExecContext(ctx,
+		"DELETE FROM client_queues WHERE created_at < ?",
+		cutoff,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"prune old queue entries: %w", err,
+		)
+	}
+
+	deleted, _ := res.RowsAffected()
+
+	return deleted, nil
+}
+
+// PruneOrphanedMessages deletes messages that are no
+// longer referenced by any client_queues row and returns
+// the number of rows removed.
+func (database *Database) PruneOrphanedMessages(
+	ctx context.Context,
+) (int64, error) {
+	res, err := database.conn.ExecContext(ctx,
+		`DELETE FROM messages WHERE id NOT IN
+		 (SELECT DISTINCT message_id
+		  FROM client_queues)`,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"prune orphaned messages: %w", err,
+		)
+	}
+
+	deleted, _ := res.RowsAffected()
+
+	return deleted, nil
+}
+
+// RotateChannelMessages enforces MAX_HISTORY per channel
+// by deleting the oldest messages beyond the limit for
+// each msg_to target. Returns the total number of rows
+// removed.
+func (database *Database) RotateChannelMessages(
+	ctx context.Context,
+	maxHistory int,
+) (int64, error) {
+	if maxHistory <= 0 {
+		return 0, nil
+	}
+
+	// Find distinct targets that have messages.
+	rows, err := database.conn.QueryContext(ctx,
+		`SELECT msg_to, COUNT(*) AS cnt
+		 FROM messages
+		 WHERE msg_to != ''
+		 GROUP BY msg_to
+		 HAVING cnt > ?`,
+		maxHistory,
+	)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"list targets for rotation: %w", err,
+		)
+	}
+
+	defer func() { _ = rows.Close() }()
+
+	type targetCount struct {
+		target string
+		count  int64
+	}
+
+	var targets []targetCount
+
+	for rows.Next() {
+		var entry targetCount
+
+		err = rows.Scan(&entry.target, &entry.count)
+		if err != nil {
+			return 0, fmt.Errorf(
+				"scan target count: %w", err,
+			)
+		}
+
+		targets = append(targets, entry)
+	}
+
+	err = rows.Err()
+	if err != nil {
+		return 0, fmt.Errorf("rows error: %w", err)
+	}
+
+	var totalDeleted int64
+
+	for _, entry := range targets {
+		res, delErr := database.conn.ExecContext(ctx,
+			`DELETE FROM messages
+			 WHERE msg_to = ?
+			   AND id NOT IN (
+			     SELECT id FROM messages
+			     WHERE msg_to = ?
+			     ORDER BY id DESC
+			     LIMIT ?
+			   )`,
+			entry.target, entry.target, maxHistory,
+		)
+		if delErr != nil {
+			return totalDeleted, fmt.Errorf(
+				"rotate messages for %s: %w",
+				entry.target, delErr,
+			)
+		}
+
+		deleted, _ := res.RowsAffected()
+		totalDeleted += deleted
+	}
+
+	return totalDeleted, nil
+}